Rectifier circuits are commonly used for converting an alternating current (AC) signal into a direct current (DC) signal. Applications that require either DC power or AC power at a different frequency initially require the 50–60 Hz three phase AC power to be rectified. The rectified DC power can then be used or processed using power conversion modules.
Some applications require DC power at a higher or lower level than the rectified DC voltage. In this situation, a power conversion module converts the DC power to the desired higher or lower DC level. When AC power at a different frequency or voltage is desired, the rectified DC power is inverted by a power conversion module to AC at the desired voltage or frequency.
In some situations, it is desirable to run a DC-AC power conversion module without using a regulated DC power supply. Certain types of power conversion modules (especially 1 MHz and up) become significantly less efficient as their DC supply voltage is increased.
There are many applications for power conversion modules that are supplied by 400VAC (common in foreign countries) or 480VAC (common in the United States) mains. These applications include RF amplifiers and RF generators. The standard practice for high frequency power conversion modules is to connect two or more lower voltage power conversion modules in series. However, if one of the series connected modules fails during operation, the whole system fails. It is also difficult to share the rectified DC input voltage evenly between the series connected power conversion modules.
When the AC supply is three phase, three AC signals and a ground and/or neutral are typically provided. When the neutral is available, the voltages that are delivered to the circuit can be phase-to-phase or phase-to-neutral. In many facilities, however, the neutral connection is not available. When no neutral connection is available, the voltages that are delivered to the circuit can be only phase-to-phase. The phase-to-neutral voltages are typically lower than the phase-to-phase voltages by a factor that is equal to √{square root over (3)}. The lower voltages allow higher efficiency in DC-AC high frequency applications and a phase-to-neutral connection would be utilized if availability of the neutral connection was guaranteed. Also, there are current restrictions for the neutral wires, and any application which utilizes a neutral connection provided by their facility is subject to those restrictions.